1. Field of the Invention
The invention relates to a fixed-echo cancelling radio altimeter for measuring low altitudes, which comprises a modulator-controlled generator for producing a linearly frequency-modulated signal in the form of identical sawteeth separated by plateaus, transmission means for transmitting to the ground a wave corresponding to the signal from said generator, receiving means for receiving the reflected wave, a mixer for forming a beat frequency signal Fb.sub.t between the transmitted and received waves, said beat frequency signal being representative of the accumulation of the fixed and mobile echoes between the signals.
2. Description of the Related Art
FM/CW radio altimeters essentially as described in the preceding paragraph are well-known in the field of distance meters in general, and more particularly for measuring the height (altitude) H above ground of an aircraft in which they are installed. If the sawteeth have fixed values, the frequency fb of the signal Fb.sub.t forms a direct measurement of the desired altitude H according to the conventional formula: ##EQU3## where: c: velocity of an electromagnetic wave in the air,
T1: duration of the sawtooth, PA1 .DELTA.F: frequency sweep of the transmitted signals (wobbling frequency).
Formula (1) is obtained, for that matter, by eliminating .tau. in the following equations: ##EQU4## .tau. being the delay between a transmitted wave and the received echo wave.
This mode of operation, which achieves a very simple structure of the radio altimeter, is very suitable for digitally processing the sampled signal Fb.sub.t by means of a time-frequncy transformation such as, for example, a Fourier transform, because the frequency fb characteristic of the echo from the ground to be isolated can generally be easily recognized in the spectrum obtained for the signal Fb.
However, a fundamental problem still resides in the technique mentioned hereinbefore or other techniques including analog processing of the signal which arises in measuring very small altitudes, typically below 6 m. In this case, the mobile echoes, that is to say, in the case of a radio altimeter the echoes from the ground, and the fixed parasitic echoes provide signals having adjacent frequencies in the beat frequency signal Fb.sub.t and comparable amplitudes. The fixed echoes are essentially those produced by the superstructures of the aircraft, more particularly the landing gear, and those echoes resulting from interference due to local oscillation in the aerial system, that is, the parasitic coupling between aerials if a radio altimeter has two aerials, or the parasitic reflection of the transmitted signal on the aerial connector if a radio altimeter has a single transmitting-receiving aerial. At altitudes exceeding 6 m, the frequencies of the components of the signal Fb.sub.t which result from mobile and fixed echoes, are far apart for the latter to be eliminated by frequency filtering. This problem becomes complicated because the position of the aerial on the fuselage of an aircraft, such as a military aircraft or specifically a missile, is important and it may be impossible to place the aerial system in an optimal way in the shelter of any superstructure of the aircraft and because the aerial system is desired to be as compact as possible. The best solution to ensure this compactness is obviously to use a single transmitting-receiving aerial, but the above parasitic reflection on the aerial is then so strong that measuring short distances is virtually made impossible, because the useful part of the signal Fb.sub.t then virtually drowned in the parasitic signal caused by the fixed echoes, the spectrum of the signal Fb.sub.t then being partly degraded with such short distances. For a two-aerial radio altimeter, to which this invention is directed, according to the prior art a decoupling of 70 dB must be maintained, that is to say, a distance of about 50 cm between the aerials in order to obtain a ratio of the useful signal to the parasitic signal caused by fixed echoes which is sufficiently high compared to the beat frequency signal at distances below 6 m.
This invention proposes to apply the principle of fixed-echo cancellation (Mobile Target Indicator or MTI) to a radio altimeter of the FM/CW type so as to clear the beat frequency signal Fb.sub.t at low altitudes. A distance measuring system of the FM/CW type applying the MTI principle is disclosed in published European Patent Application No. 0 099 160. However, said European Application differs from the present invention and leads to a different solution because the system therein is a radar for detecting from various moving targets, generally at high speeds, distances to be measured which are large, making it possible to use a single aerial. In addition, that system concerns resolving the problem due to the Doppler effect caused by the moving targets, which problem does not occur for a low-altitude radio altimeter. In the moving target indication system of said European Patent Application it is provided, for echo cancellation, to make a distinction between two successive echoes, that is, to use the difference between the beat frequency signals of two consecutive sawteeth as a useful beat frequency signal.